Getting Started with Javassist
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## 5. Introspection and customization

`CtClass` provides methods for introspection. The introspective ability of Javassist is compatible with that of the Java reflection API. `CtClass` provides `getName()`, `getSuperclass()`, `getMethods()`, and so on. `CtClass` also provides methods for modifying a class definition. It allows to add a new field, constructor, and method. Instrumenting a method body is also possible.

Methods are represented by `CtMethod` objects. `CtMethod` provides several methods for modifying the definition of the method. Note that if a method is inherited from a super class, then the same `CtMethod` object that represents the inherited method represents the method declared in that super class. A `CtMethod` object corresponds to every method declaration.

For example, if class `Point` declares method `move()` and a subclass `ColorPoint` of `Point` does not override `move()`, the two `move()` methods declared in `Point` and inherited in `ColorPoint` are represented by the identical `CtMethod` object. If the method definition represented by this `CtMethod` object is modified, the modification is reflected on both the methods. If you want to modify only the `move()` method in `ColorPoint`, you first have to add to `ColorPoint` a copy of the `CtMethod` object representing `move()` in `Point`. A copy of the the `CtMethod` object can be obtained by `CtNewMethod.copy()`.

Javassist does not allow to remove a method or field, but it allows to change the name. So if a method is not necessary any more, it should be renamed and changed to be a private method by calling `setName()` and `setModifiers()` declared in `CtMethod`.

Javassist does not allow to add an extra parameter to an existing method, either. Instead of doing that, a new method receiving the extra parameter as well as the other parameters should be added to the same class. For example, if you want to add an extra `int` parameter `newZ` to a method:

    void move(int newX, int newY) { x = newX; y = newY; }

in a `Point` class, then you should add the following method to the `Point` class:

    void move(int newX, int newY, int newZ) {
        // do what you want with newZ.
        move(newX, newY);
    }

Javassist also provides low-level API for directly editing a raw class file. For example, `getClassFile()` in `CtClass` returns a `ClassFile` object representing a raw class file. `getMethodInfo()` in `CtMethod` returns a `MethodInfo` object representing a `method_info` structure included in a class file. The low-level API uses the vocabulary from the Java Virtual machine specification. The users must have the knowledge about class files and bytecode. For more details, the users should see the `javassist.bytecode` package.

### 5.1 Inserting source text at the beginning/end of a method body

`CtMethod` and `CtConstructor` provide methods `insertBefore()`, `insertAfter()`, and `addCatch()`. They are used for inserting a code fragment into the body of an existing method. The users can specify those code fragments with source text written in Java. Javassist includes a simple Java compiler for processing source text. It receives source text written in Java and compiles it into Java bytecode, which will be inlined into a method body.

The methods `insertBefore()`, `insertAfter()`, and `addCatch()` receives a `String` object representing a statement or a block. A statement is a single control structure like `if` and `while` or an expression ending with a semi colon (`;`). A block is a set of statements surrounded with braces `{}`. Hence each of the following lines is an example of valid statement or block:

    System.out.println("Hello");
    { System.out.println("Hello"); }
    if (i < 0) { i = -i; }

The statement and the block can refer to fields and methods. However, they cannot refer to local variables declared in the method that they are inserted into. They can refer to the parameters to the method although they must use different names `$0`, `$1`, `$2`, ... described below. Declaring a local variable in the block is allowed.

The `String` object passed to the methods `insertBefore()`, `insertAfter()`, and `addCatch()` are compiled by the compiler included in Javassist. Since the compiler supports language extensions, several identifiers starting with `$` have special meaning:

`$0`, `$1`, `$2`, ... | Actual parameters
`$args` | An array of parameters. The type of `$args` is `Object[]`.
`$$` | All actual parameters. For example, `m($$)` is equivalent to `m($1,$2,`...`)`
`$cflow(`...`)` | `cflow` variable
`$r` | The result type. It is used in a cast expression.
`$w` | The wrapper type. It is used in a cast expression.
`$_` | The resulting value
`$sig` | An array of `java.lang.Class` objects representing the formal parameter types.
`$type` | A `java.lang.Class` object representing the formal result type.
`$class` | A `java.lang.Class` object representing the class currently edited.

#### $0, $1, $2, ...

The parameters passed to the methods `insertBefore()`, `insertAfter()`, and `addCatch()` are accessible with `$0`, `$1`, `$2`, ... instead of the original parameter names. `$1` represents the first parameter, `$2` represents the second parameter, and so on. The types of those variables are identical to the parameter types. `$0` is equivalent to `this`. If the method is static, `$0` is not available.

These variables are used as following. Suppose that a class `Point`:

    class Point {
        int x, y;
        void move(int dx, int dy) { x += dx; y += dy; }
    }

To print the values of `dx` and `dy` whenever the method `move()` is called, execute this program:

    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("Point");
    CtMethod m = cc.getDeclaredMethod("move");
    m.insertBefore("{ System.out.println($1); System.out.println($2); }");
    cc.writeFile();

Note that the source text passed to `insertBefore()` is surrounded with braces `{}`. `insertBefore()` accepts only a single statement or a block surrounded with braces.

The definition of the class `Point` after the modification is like this:

    class Point {
        int x, y;
        void move(int dx, int dy) {
            { System.out.println(dx); System.out.println(dy); }
            x += dx; y += dy;
        }
    }

`$1` and `$2` are replaced with `dx` and `dy`, respectively.

`$1`, `$2`, `$3` ... are updatable. If a new value is assigend to one of those variables, then the value of the parameter represented by that variable is also updated.

#### $args

The variable `$args` represents an array of all the parameters. The type of that variable is an array of class `Object`. If a parameter type is a primitive type such as `int`, then the parameter value is converted into a wrapper object such as `java.lang.Integer` to store in `$args`. Thus, `$args[0]` is equivalent to `$1` unless the type of the first parameter is a primitive type. Note that `$args[0]` is not equivalent to `$0`; `$0` represents `this`.

If an array of `Object` is assigned to `$args`, then each element of that array is assigned to each parameter. If a parameter type is a primitive type, the type of the corresponding element must be a wrapper type. The value is converted from the wrapper type to the primitive type before it is assigned to the parameter.

#### $$

The variable `$$` is abbreviation of a list of all the parameters separated by commas. For example, if the number of the parameters to method `move()` is three, then

    move($$)

is equivalent to this:

    move($1, $2, $3)

If `move()` does not take any parameters, then `move($$)` is equivalent to `move()`.

`$$` can be used with another method. If you write an expression:

    exMove($$, context)

then this expression is equivalent to:

    exMove($1, $2, $3, context)

Note that `$$` enables generic notation of method call with respect to the number of parameters. It is typically used with `$proceed` shown later.

#### $cflow

`$cflow` means "control flow". This read-only variable returns the depth of the recursive calls to a specific method.

Suppose that the method shown below is represented by a `CtMethod` object `cm`:

    int fact(int n) {
        if (n <= 1)
            return n;
        else
            return n * fact(n - 1);
    }

To use `$cflow`, first declare that `$cflow` is used for monitoring calls to the method `fact()`:

    CtMethod cm = ...;
    cm.useCflow("fact");

The parameter to `useCflow()` is the identifier of the declared `$cflow` variable. Any valid Java name can be used as the identifier. Since the identifier can also include `.` (dot), for example, `"my.Test.fact"` is a valid identifier.

Then, `$cflow(fact)` represents the depth of the recursive calls to the method specified by `cm`. The value of `$cflow(fact)` is 0 (zero) when the method is first called whereas it is 1 when the method is recursively called within the method. For example,

    cm.insertBefore("if ($cflow(fact) == 0)"
                  + "    System.out.println(\"fact \" + $1);");

translates the method `fact()` so that it shows the parameter. Since the value of `$cflow(fact)` is checked, the method `fact()` does not show the parameter if it is recursively called within `fact()`.

The value of `$cflow` is the number of stack frames associated with the specified method `cm` under the current topmost stack frame for the current thread. `$cflow` is also accessible within a method different from the specified method `cm`.

#### $r

`$r` represents the result type (return type) of the method. It must be used as the cast type in a cast expression. For example, this is a typical use:

    Object result = ... ;
    $_ = ($r)result;

If the result type is a primitive type, then `($r)` follows special semantics. First, if the operand type of the cast expression is a primitive type, `($r)` works as a normal cast operator to the result type. On the other hand, if the operand type is a wrapper type, `($r)` converts from the wrapper type to the result type. For example, if the result type is `int`, then `($r)` converts from `java.lang.Integer` to `int`.

If the result type is `void`, then `($r)` does not convert a type; it does nothing. However, if the operand is a call to a `void` method, then `($r)` results in `null`. For example, if the result type is `void` and `foo()` is a `void` method, then

    $_ = ($r)foo();

is a valid statement.

The cast operator `($r)` is also useful in a `return` statement. Even if the result type is `void`, the following `return` statement is valid:

    return ($r)result;

Here, `result` is some local variable. Since `($r)` is specified, the resulting value is discarded. This `return` statement is regarded as the equivalent of the `return` statement without a resulting value:

    return;

#### $w

`$w` represents a wrapper type. It must be used as the cast type in a cast expression. `($w)` converts from a primitive type to the corresponding wrapper type. The following code is an example:

    Integer i = ($w)5;

The selected wrapper type depends on the type of the expression following `($w)`. If the type of the expression is `double`, then the wrapper type is `java.lang.Double`.

If the type of the expression following `($w)` is not a primitive type, then `($w)` does nothing.

#### $_

`insertAfter()` in `CtMethod` and `CtConstructor` inserts the compiled code at the end of the method. In the statement given to `insertAfter()`, not only the variables shown above such as `$0`, `$1`, ... but also `$_` is available.

The variable `$_` represents the resulting value of the method. The type of that variable is the type of the result type (the return type) of the method. If the result type is `void`, then the type of `$_` is `Object` and the value of `$_` is `null`.

Although the compiled code inserted by `insertAfter()` is executed just before the control normally returns from the method, it can be also executed when an exception is thrown from the method. To execute it when an exception is thrown, the second parameter `asFinally` to `insertAfter()` must be `true`.

If an exception is thrown, the compiled code inserted by `insertAfter()` is executed as a `finally` clause. The value of `$_` is `0` or `null` in the compiled code. After the execution of the compiled code terminates, the exception originally thrown is re-thrown to the caller. Note that the value of `$_` is never thrown to the caller; it is rather discarded.

#### $sig

The value of `$sig` is an array of `java.lang.Class` objects that represent the formal parameter types in declaration order.

#### $type

The value of `$type` is an `java.lang.Class` object representing the formal type of the result value. This variable is available only in `insertAfter()` in `CtMethod` and `CtConstructor`.

#### $class

The value of `$class` is an `java.lang.Class` object representing the class in which the edited method is declared.

#### addCatch()

`addCatch()` inserts a code fragment into a method body so that the code fragment is executed when the method body throws an exception and the control returns to the caller. In the source text representing the inserted code fragment, the exception value is referred to with the special variable `$e`.

For example, this program:

    CtMethod m = ...;
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ System.out.println($e); throw $e; }", etype);

translates the method body represented by `m` into something like this:

    try {
        the original method body
    }
    catch (java.io.IOException e) {
        System.out.println(e);
        throw e;
    }

Note that the inserted code fragment must end with a `throw` or `return` statement.

### 5.2 Modifying a method body

`CtMethod` and `CtConstructor` provide `setBody()` for substituting a whole method body. They compile the given source text into Java bytecode and substitutes it for the original method body. If the given source text is `null`, the substituted body includes only a `return` statement, which returns zero or null unless the result type is `void`.

In the source text given to `setBody()`, the identifiers starting with `$` have special meaning

`$0`, `$1`, `$2`, ... | Actual parameters
`$args` | An array of parameters. The type of `$args` is `Object[]`.
`$$` | All actual parameters.
`$cflow(`...`)` | `cflow` variable
`$r` | The result type. It is used in a cast expression.
`$w` | The wrapper type. It is used in a cast expression.
`$sig` | An array of `java.lang.Class` objects representing the formal parameter types.
`$type` | A `java.lang.Class` object representing the formal result type.
`$class` | A `java.lang.Class` object representing the class currently edited.

Note that `$_` is not available.

Javassist allows modifying only an expression included in a method body. `javassist.expr.ExprEditor` is a class for replacing an expression in a method body. The users can define a subclass of `ExprEditor` to specify how an expression is modified.

To run an `ExprEditor` object, the users must call `instrument()` in `CtMethod` or `CtClass`. For example,

    CtMethod cm = ... ;
    cm.instrument(
        new ExprEditor() {
            public void edit(MethodCall m)
                          throws CannotCompileException
            {
                if (m.getClassName().equals("Point")
                              && m.getMethodName().equals("move"))
                    m.replace("{ $1 = 0; $_ = $proceed($$); }");
            }
        });

searches the method body represented by `cm` and replaces all calls to `move()` in class `Point` with a block:

    { $1 = 0; $_ = $proceed($$); }

so that the first parameter to `move()` is always 0. Note that the substituted code is not an expression but a statement or a block.

The method `instrument()` searches a method body. If it finds an expression such as a method call, field access, and object creation, then it calls `edit()` on the given `ExprEditor` object. The parameter to `edit()` is an object representing the found expression. The `edit()` method can inspect and replace the expression through that object.

Calling `replace()` on the parameter to `edit()` substitutes the given statement or block for the expression. If the given block is an empty block, that is, if `replace("{}")` is executed, then the expression is removed from the method body. If you want to insert a statement (or a block) before/after the expression, a block like the following should be passed to `replace()`:

    { before-statements;
      $_ = $proceed($$);
      after-statements; }

whichever the expression is either a method call, field access, object creation, or others. The second statement could be:

    $_ = $proceed();

if the expression is read access, or

    $proceed($$);

if the expression is write access.

#### javassist.expr.MethodCall

A `MethodCall` object represents a method call. The method `replace()` in `MethodCall` substitutes a statement or a block for the method call. It receives source text representing the substitued statement or block, in which the identifiers starting with `$` have special meaning as in the source text passed to `insertBefore()`.

`$0` | The target object of the method call. This is not equivalent to `this`, which represents the caller-side `this` object. `$0` is `null` if the method is static.
`$1`, `$2`, ... | The parameters of the method call.
`$_` | The resulting value of the method call.
`$r` | The result type of the method call.
`$class` | A `java.lang.Class` object representing the class declaring the method.
`$sig` | An array of `java.lang.Class` objects representing the formal parameter types.
`$type` | A `java.lang.Class` object representing the formal result type.
`$proceed` | The name of the method originally called in the expression.

Here the method call means the one represented by the `MethodCall` object.

The other identifiers such as `$w`, `$args` and `$$` are also available.

Unless the result type of the method call is `void`, a value must be assigned to `$_` in the source text and the type of `$_` is the result type. If the result type is `void`, the type of `$_` is `Object` and the value assigned to `$_` is ignored.

`$proceed` is not a `String` value but special syntax. It must be followed by an argument list surrounded by parentheses `( )`.

#### javassist.expr.FieldAccess

A `FieldAccess` object represents field access. The method `edit()` in `ExprEditor` receives this object if field access is found. The method `replace()` in `FieldAccess` receives source text representing the substitued statement or block for the field access. In the source text, the identifiers starting with `$` have also special meaning:

`$0` | The object containing the field accessed by the expression. This is not equivalent to `this`. `this` represents the object that the method including the expression is invoked on. `$0` is `null` if the field is static.
`$1` | The value that would be stored in the field if the expression is write access. Otherwise, `$1` is not available.
`$_` | The resulting value of the field access if the expression is read access. Otherwise, the value stored in `$_` is discarded.
`$r` | The type of the field if the expression is read access. Otherwise, `$r` is `void`.
`$class` | A `java.lang.Class` object representing the class declaring the field.
`$type` | A `java.lang.Class` object representing the field type.
`$proceed` | The name of a virtual method executing the original field access. .

The other identifiers such as `$w`, `$args` and `$$` are also available.

If the expression is read access, a value must be assigned to `$_` in the source text. The type of `$_` is the type of the field.

#### javassist.expr.NewExpr

A `NewExpr` object represents object creation with the `new` operator. The method `edit()` in `ExprEditor` receives this object if object creation is found. The method `replace()` in `NewExpr` receives source text representing the substitued statement or block for the object creation. In the source text, the identifiers starting with `$` have also special meaning:

`$0` | `null`.
`$1`, `$2`, ... | The parameters to the constructor.
`$_` | The resulting value of the object creation. A newly created object must be stored in this variable.
`$r` | The type of the created object.
`$class` | A `java.lang.Class` object representing the class of the created object.
`$sig` | An array of `java.lang.Class` objects representing the formal parameter types.
`$proceed` | The name of a virtual method executing the original object creation. .

The other identifiers such as `$w`, `$args` and `$$` are also available.

#### javassist.expr.Instanceof

A `Instanceof` object represents an `instanceof` expression. The method `edit()` in `ExprEditor` receives this object if an instanceof expression is found. The method `replace()` in `Instanceof` receives source text representing the substitued statement or block for the expression. In the source text, the identifiers starting with `$` have also special meaning:

`$0` | `null`.
`$1` | The value on the left hand side of the original `instanceof` operator.
`$_` | The resulting value of the expression. The type of `$_` is `boolean`.
`$r` | The type on the right hand side of the `instanceof` operator.
`$type` | A `java.lang.Class` object representing the type on the right hand side of the `instanceof` operator.
`$proceed` | The name of a virtual method executing the original `instanceof` expression. It takes one parameter (the type is `java.lang.Object`) and returns true if the parameter value is an instance of the type on the right hand side of the original `instanceof` operator. Otherwise, it returns false.

The other identifiers such as `$w`, `$args` and `$$` are also available.

#### javassist.expr.Cast

A `Cast` object represents an expression for explicit type casting. The method `edit()` in `ExprEditor` receives this object if explicit type casting is found. The method `replace()` in `Cast` receives source text representing the substitued statement or block for the expression. In the source text, the identifiers starting with `$` have also special meaning:

`$0` | `null`.
`$1` | The value the type of which is explicitly cast.
`$_` | The resulting value of the expression. The type of `$_` is the same as the type after the explicit casting, that is, the type surrounded by `( )`.
`$r` | the type after the explicit casting, or the type surrounded by `( )`.
`$type` | A `java.lang.Class` object representing the same type as `$r`.
`$proceed` | The name of a virtual method executing the original type casting. It takes one parameter of the type `java.lang.Object` and returns it after the explicit type casting specified by the original expression.

The other identifiers such as `$w`, `$args` and `$$` are also available.

#### javassist.expr.Handler

A `Handler` object represents a `catch` clause of `try-catch` statement. The method `edit()` in `ExprEditor` receives this object if a `catch` is found. The method `insertBefore()` in `Handler` compiles the received source text and inserts it at the beginning of the `catch` clause. In the source text, the identifiers starting with `$` have special meaning:

`$1` | The exception object caught by the `catch` clause.
`$r` | the type of the exception caught by the `catch` clause. It is used in a cast expression.
`$w` | The wrapper type. It is used in a cast expression.
`$type` | A `java.lang.Class` object representing the type of the exception caught by the `catch` clause.

If a new exception object is assigned to `$1`, it is passed to the original `catch` clause as the caught exception.

### 5.3 Adding a new method or field

#### Adding a method

Javassist allows the users to create a new method and constructor from scratch. `CtNewMethod` and `CtNewConstructor` provide several factory methods, which are static methods for creating `CtMethod` or `CtConstructor` objects. Especially, `make()` creates a `CtMethod` or `CtConstructor` object from the given source text.

For example, this program:

    CtClass point = ClassPool.getDefault().get("Point");
    CtMethod m = CtNewMethod.make(
                     "public int xmove(int dx) { x += dx; }",
                     point);
    point.addMethod(m);

adds a public method `xmove()` to class `Point`. In this example, `x` is a `int` field in the class `Point`.

The source text passed to `make()` can include the identifiers starting with `$` except `$_` as in `setBody()`. It can also include `$proceed` if the target object and the target method name are also given to `make()`. For example,

    CtClass point = ClassPool.getDefault().get("Point");
    CtMethod m = CtNewMethod.make(
                     "public int ymove(int dy) { $proceed(0, dy); }",
                     point, "this", "move");

this program creates a method `ymove()` defined below:

    public int ymove(int dy) { this.move(0, dy); }

Note that `$proceed` has been replaced with `this.move`.

Javassist provides another way to add a new method. You can first create an abstract method and later give it a method body:

    CtClass cc = ... ;
    CtMethod m = new CtMethod(CtClass.intType, "move",
                              new CtClass[] { CtClass.intType }, cc);
    cc.addMethod(m);
    m.setBody("{ x += $1; }");
    cc.setModifiers(cc.getModifiers() & ~Modifier.ABSTRACT);

Since Javassist makes a class abstract if an abstract method is added to the class, you have to explicitly change the class back to a non-abstract one after calling `setBody()`.

#### Mutual recursive methods

Javassist cannot compile a method if it calls another method that has not been added to a class. (Javassist can compile a method that calls itself recursively.) To add mutual recursive methods to a class, you need a trick shown below. Suppose that you want to add methods `m()` and `n()` to a class represented by `cc`:

    CtClass cc = ... ;
    CtMethod m = CtNewMethod.make("public abstract int m(int i);", cc);
    CtMethod n = CtNewMethod.make("public abstract int n(int i);", cc);
    cc.addMethod(m);
    cc.addMethod(n);
    m.setBody("{ return ($1 <= 0) ? 1 : (n($1 - 1) * $1); }");
    n.setBody("{ return m($1); }");
    cc.setModifiers(cc.getModifiers() & ~Modifier.ABSTRACT);

You must first make two abstract methods and add them to the class. Then you can give the method bodies to these methods even if the method bodies include method calls to each other. Finally you must change the class to a not-abstract class since `addMethod()` automatically changes a class into an abstract one if an abstract method is added.

#### Adding a field

Javassist also allows the users to create a new field.

    CtClass point = ClassPool.getDefault().get("Point");
    CtField f = new CtField(CtClass.intType, "z", point);
    point.addField(f);

This program adds a field named `z` to class `Point`.

If the initial value of the added field must be specified, the program shown above must be modified into:

    CtClass point = ClassPool.getDefault().get("Point");
    CtField f = new CtField(CtClass.intType, "z", point);
    point.addField(f, "0");    // initial value is 0.

Now, the method `addField()` receives the second parameter, which is the source text representing an expression computing the initial value. This source text can be any Java expression if the result type of the expression matches the type of the field. Note that an expression does not end with a semi colon (`;`).

### 5.4 Limitations

In the current implementation, the Java compiler included in Javassist has several limitations with respect to the language that the compiler can accept. Those limitations are:

- The `.class` notation is not supported. Use the method `Class.forName()`. In regular Java, an expression `Point.class` means a `Class` object representing the `Point` class. This notation is not available.

- Array initializers, a comma-separated list of expressions enclosed by braces `{` and `}`, are not supported.

- Inner classes or anonymous classes are not supported.

- `switch` or `synchronized` statements are not supported yet.

- Labeled `continue` and `break` statements are not supported.

- The `finally` clause following `try` and `catch` clauses is not supported.

- The compiler does not correctly implement the Java method dispatch algorithm. The compiler may confuse if methods defined in a class have the same name but take different parameter lists.

- The users are recommended to use `#` as the separator between a class name and a static method or field name. For example, in regular Java,

      javassist.CtClass.intType.getName()

  calls a method `getName()` on the object indicated by the static field `intType` in `javassist.CtClass`. In Javassist, the users can write the expression shown above but they are recommended to write:

      javassist.CtClass#intType.getName()

  so that the compiler can quickly parse the expression.

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  Copyright (C) 2000-2003 by Shigeru Chiba, All rights reserved.